2-D Molecular Alloy Ru–M (M = Cu, Ag, and Au) Carbonyl Clusters: Synthesis, Molecular Structure, Catalysis, and Computational Studies

The reactions of [HRu3(CO)11]− (1) with M(I) (M = Cu, Ag, and Au) compounds such as [Cu(CH3CN)4][BF4], AgNO3, and Au(Et2S)Cl afford the 2-D molecular alloy clusters [CuRu6(CO)22]− (2), [AgRu6(CO)22]− (3), and [AuRu5(CO)19]− (4), respectively. The reactions of 2–4 with PPh3 result in mixtures of products, among which [Cu2Ru8(CO)26]2– (5), Ru4(CO)12(CuPPh3)4 (6), Ru4(CO)12(AgPPh3)4 (7), Ru(CO)3(PPh3)2 (8), and HRu3(OH)(CO)7(PPh3)3 (9) have been isolated and characterized. The molecular structures of 2–6 and 9 have been determined by single-crystal X-ray diffraction. The metal–metal bonding within 2–5 has been computationally investigated by density functional theory methods. In addition, the [NEt4]+ salts of 2–4 have been tested as catalyst precursors for transfer hydrogenation on the model substrate 4-fluoroacetophenone using iPrOH as a solvent and a hydrogen source.


INTRODUCTION
Molecular clusters usually adopt 3-D structures that consist of tridimensional metal cores, such as tetrahedron, octahedron, icosahedron, and larger polyhedral, as well as more complex and irregular structures. 1,2 Further growth in a 3-D mode results in molecular nanoclusters and larger metal nanoparticles. 3−10 Alternatively, molecular clusters may adopt 2-D structures that consist of a planar or almost planar arrangement of metal atoms. 11,12 Even if 2-D clusters are rarer than 3-D ones, representative examples are found within heterometallic (alloy) molecular carbonyl clusters such as [M 3 Fe 3 (CO) 12 ] 3− (M = Cu, Ag, and Au), 13,14 [M 4 Fe 4 (CO) 16 ] 4− (M = Ag and Au), 15,16 [M 5 Fe 4 (CO) 16 ] 3− (M = Cu, Ag, and Au), 15−17 [Os 9 Hg 3 (CO) 30 ], 18 and [IrRu 6 (CO) 23 ] − . 19 It should be mentioned that a few cases of the 1-D growth path of carbonyl clusters have been reported, 20 being homoleptic and heteroleptic Chini clusters the most astonishing examples. 21−23 Compared to 3-D clusters, the nuclearity of 2-D clusters reported so far is rather limited. Nonetheless, they are attractive from a structural point of view as models of metal surfaces and monolayers. 24−26 Heterometallic complexes and clusters containing polar metal−metal interactions are attracting interest for the activation of small molecules and catalytic applications. 27−31 Examples include C−H functionalization, carbonylation, hydrogenation, 32 as well as ammonia-borane dehydrogenation. 33 Like ammonia-borane, alcohols such as iso-propanol also contain at the same time both hydridic-like (C−H) and protic (O−H) hydrogens, suitable for activation via interaction with a polar metal−metal bond. Thus, heterometallic clusters such as Ru−M (M = Cu, Ag, and Au) might be active as catalysts for transfer hydrogenation reactions. 34 Within this framework, herein, we report the synthesis of the 2-D molecular alloy carbonyl clusters [CuRu 6 (CO) 22 ] − , [AgRu 6 (CO) 22 ] − , and [AuRu 5 (CO) 19 ] − . Their molecular structures have been determined by single-crystal X-ray diffraction (SC-XRD), and the metal−metal bonding has been analyzed by computational methods. Moreover, some preliminary results on their use as pre-catalysts in transfer hydrogenation reactions are reported. ] display only terminal carbonyls, with no evidence of bridging or semibridging ligands. It may be that the M···C(O) contacts are present only in the solid state due to packing effects, or that such interactions are weak and do not affect the IR spectra. 36 40 The larger size of Ag as compared to Cu has two further consequences. First, the inter-triangular Ru−Ru contact of 3 [2.9925(4) Å] is longer than that of 2 [2.9304(15) Å], whereas the intra-triangular Ru−Ru bonding distances of the two   The density functional theory (DFT)-optimized structure of 2 is in good agreement with the experimental data [root-meansquare deviation (RMSD) = 0.359 Å]. The shortest Cu···C distance observed in the X-ray structure is elongated by about 0.48 Å in the computed geometry, and no bond critical point (b.c.p.) was found between the copper center and the terminal carbonyl ligands; therefore, the Cu···C(O) contacts are probably due to packing effects. On the other hand, b.c.p.'s were found for all the Cu−Ru and Ru−Ru bonds, as observable in Figure 3. Selected data are collected in Table 2 and summarized on the basis of the approximate C 2 symmetry of the optimized geometry (R = 0.116). In all cases, the negative values of energy density (E) and the positive values of the Laplacian of electron density (∇ 2 ρ) at b.c.p.'s are in line with Bianchi's classification of metal−metal bonds. 41 The different strengths of the Cu−Ru interactions are highlighted in particular by the Wiberg analysis ( Table 2 As for 2, the DFT-optimized structure of 3 is in good agreement with the experimental data (RMSD = 0.357 Å). In this case, the optimized geometry has a regular C 2 symmetry (R = 0.000), also revealed by the data computed for the M−M b.c.p.'s and by the Wiberg bond orders, summarized in Table 3. As for 2, no b.c.p. was found between the carbonyl ligands and the coinage metal. The DFT-optimized geometry of 3, including M−M b.c.p.'s, is shown in Figure 4. On the basis of the ρ and V data at b.c.p.'s, Ag(1)−Ru(1)/Ag(1)−Ru (6) bonds are stronger than those of Ag(1)−Ru(3)/Ag(1)−Ru(4) (see Scheme 2 for labeling). Accordingly, the Ag(1)−Ru(1)/ Ag(1)−Ru(6) bond order is 0.412, while that of Ag(1)− Ru(3)/Ag(1)−Ru(4) is 0.354. The Ru−Ru bond strengths follow the order previously described for 2, that is, Ru(2)− Ru(3)/Ru(4)−Ru(5) > Ru(1)−Ru(2)/Ru(5)−Ru(6) ≈ Ru(3)−Ru(4) > Ru(1)−Ru(3)/Ru(4)−Ru (6). The Ru(1)− Ru(3)/Ru(4)−Ru(6) bonds appear weaker in 3 with respect  [MRu 6 (CO) 22 ] − may be viewed as composed of a [Ru 6 (CO) 22 ] 2− anionic unit, which acts as a tetradentate ligand via four Ru atoms toward a single M + cation. In view of this description, we can devise a possible formal mechanism for the formation of [MRu 6 (CO) 22  Possible intermediate species involved in the formation of 2 from 1 were computationally investigated. In particular, the interaction of two 1 clusters with a copper monocation afforded the compound [Cu(μ-H) 2 {Ru 3 (CO) 11 } 2 ] − depicted in Figure 5 as the most stable stationary point. [Cu(μ-H) 2 {Ru 3 (CO) 11 } 2 ] − has an approximate C 2 symmetry (R = 0.047). The trinuclear {Ru 3 } fragments maintain the triangular arrangement, and both the hydrides bridge the Cu and one of the Ru centers. Two Cu−Ru bonds are also observable, and their presence was confirmed by the AIM and Wiberg analyses. The computed energy variation (sum of electronic energy and nuclear repulsion) for the reaction (1) is −11.8 kcal mol −1 (−13.7 kcal mol −1 including CH 2 Cl 2 as continuous medium). (1) Electron density values at M−H and M−M b.c.p.'s are reported in Figure 5. Furthermore, AIM and Wiberg data are collected in Table S1 in the Supporting Information. The presence of Cu−H bonds was confirmed by all the computational analyses, but it is worth noting that the average Ru−H bond order is 0.556, much greater than the value related to the Cu−H interactions, 0.211. As for 2 and 3, the RMSD of the DFT-optimized structure with respect to X-ray data is small (0.280 Å). The AIM analysis was unable to locate b.c.p.'s between most of the ruthenium centers, with the exception of the Ru(3)−Ru(3A) bond (see Scheme 4 for labeling). On the contrary, b.c.p.'s were found for all the Ru−Au bonds. Data are summarized in Table 5 Table 2. Selected Average Computed Data (a.u.) at Metal−Metal b.c.p.'s for 2 (ρ = Electron Density; V = Potential Energy Density; E = Energy Density; ∇ 2 ρ = Laplacian of Electron Density) and Wiberg Bond Orders  Table 5). The strength order of the Ru−Au bonds, according to the data reported in Table 5,

Synthesis and Molecular
. It is, however, worth noting that the ρ and V values are quite similar   Table 6). The Wiberg analysis therefore suggests that the formation of the Ru−Au bonds causes a weakening of the Ru−Ru interactions. The average Hirshfeld 0.14 α CO(2) 0.51 mean deviation from AuRu 5 least-square plane 0.0781 Au (1) The anion 5 ( Figure 9) may be viewed as composed of two tetrahedral [Ru 4 (CO) 13 19 ] 2− (ρ = Electron Density; V = Potential Energy Density; E = Energy Density; ∇ 2 ρ = Laplacian of Electron Density) and Wiberg Bond Orders The AIM analysis on the DFT-optimized structure of 5 (RMSD deviation with respect to the X-ray data equal to 0.404 Å) revealed the presence of (3,−1) b.c.p.'s in line with M−M bonds (see Figure 10 and Table S2 in the Supporting Information for selected AIM data). In particular, one b.c.p. between the two Cu atoms was localized. Four Cu−Ru b.c.p.'s are present for each copper center, three with one {Ru 4 } tetrahedron and one with the other {Ru 4 }. Five Ru−Ru b.c.p.'s were localized instead of the six expected for a Ru 4 tetrahedron. In particular, the software was unable to find (3,−1) b.c.p.'s between Ru(1) and Ru(4) and between Ru (5) and Ru (8), as observable in Figure 10. All the expected bonds were instead found using the Wiberg analysis, with a Cu−Cu bond order of 0.194, Cu−Ru bond orders between 0.245 and 0.337, and Ru−Ru bond orders between 0.398 and 0.575.
Compound 6 is composed of a Ru 4 tetrahedron whose four triangular faces are capped by four CuPPh 3 groups ( Figure 11).  43 6 is related to H 4 Ru 4 (CO) 12 , 44 even though the hydride ligands are edge bridging, whereas the Cu(I) fragments are face capping.
The reaction of 3 with PPh 3 is very similar to that described above in the case of 2. In particular, a neutral product is extracted at the end of the reaction in toluene, which shows an IR spectrum very similar to 6. Thus, the product has been tentatively formulated as Ru 4 (CO) 12 (AgPPh 3 ) 4 (7).
Compound 4 reacts only with an excess of PPh 3 , affording mixtures of Ru(CO) 3 (PPh 3 ) 2 (8) and HRu 3 (OH)-(CO) 7 (PPh 3 ) 3 (9). The yields of 9 may be improved after addition of some water to the reaction mixture. This is in keeping with the fact that the hydride and hydroxide ligands originate from H 2 O dissociation into H + and OH − . The molecular structure of 9 has been ascertained by SC-XRD ( Figure 12). It consists of a Ru 3 triangle where one Ru−Ru edge is bridged by μ-H and μ-OH ligands. All the CO ligands are terminal and, in addition, there is one PPh 3 ligand on each Ru atom.
The contemporary presence of μ-H and μ-OH ligands on the same M−M edge has been previously found in other dimers and clusters, such as HOs 3 (OH)(CO) 10 , HOs 3 (OH)-(CO) 8 Table 7). The model substrate employed was 4-fluoroacetophenone, and the reaction was monitored by 19 F NMR spectroscopy. Since 4-F-α-methylbenzylalcohol [1-(4-fluorophenyl)ethan-1-ol] was the only product observed, only conversion was analyzed. The catalytic tests were performed employing 1 or 2.5% mol of catalyst precursor per mol of the substrate at a i PrOH refluxing temperature (82°C). Tests were carried out both in the absence and in the presence of a base (KO t Bu, 10% mol/mol with respect to the substrate). All catalytic tests have been carried out at least three times using different cluster catalyst precursor batches (including crystalline batches), resulting in highly reproducible results. This seems to exclude that what is being seen is catalysis by trace impurities.   The conversion after 24 h was observed in the range of 26− 95%, suggesting some catalytic activity under all the experimental conditions considered. The conversion after 5 h was considerably lower (5−40%), indicating a long induction period. Such a long period is likely to be required in order to transform the catalyst precursors 2−4 into the active species. As described below, spectroscopic (IR, 1 H NMR, and ESI-MS) analyses performed on the reaction mixtures at the end of the catalytic processes clearly indicate that mixtures of carbonyl clusters, including hydride carbonyl clusters, are present, ruling out cluster breakdown to mononuclear complexes or nanoparticles.
As expected, conversion increased by increasing the catalyst load for all three clusters, both in the absence and presence of the base. In the case of 2, the addition of the base significantly increased the conversion at both catalyst loads. In contrast, in the case of 3 and 4, the positive effect of base addition was significant at 1% mol/mol catalyst load, whereas it was almost   General conditions: catalyst (3 or 7.5 μmol, 1% or 2.5% mol/mol), i PrOH (5 mL), KO t Bu (10 mol % when added), and 4-fluoroacetophenone (36.5 μL, 300 μmol), T = 82°C, N 2 atmosphere; the conversions were determined by 19 F NMR spectroscopy. All entries are the average of three independent catalytic runs.

Inorganic Chemistry
pubs.acs.org/IC Article negligible with a higher catalyst load. The activity as a catalyst precursor decreases in the order of 3 > 2 > 4. Also, the effect of the base on the conversion after 5 h is different for the three clusters. In particular, in the case of the best catalyst precursor 3, addition of the base has a negligible (or even negative effect) on the conversion after 5 h. This further corroborates the opinion that activation does not involve cluster breakdown but cluster transformation. For comparison, homometallic cluster 1 was employed as a catalyst precursor under similar experimental conditions. The conversions measured for 1 in the absence of a base were rather good, whereas addition of the base had a strong detrimental effect, at a difference from 2−4. The effect of increasing the catalyst load of 1 from 1 to 2.5% in the absence of the base had a negligible effect, also in this case, showing a different behavior as compared to 2−4. The fact that the effects of the catalyst load and addition of the base were very different in the case of 2−4 and 1 suggests a different mechanism for the activation and/or catalysis in the case of heterometallic Ru−M clusters as compared to the homometallic one. Even if it is not possible at the moment to depict a mechanism, there is spectroscopic evidence of the fact that cluster breakdown to mononuclear species or nanoparticles does not occur both for heterometallic and homometallic precursors.
Control experiments have been carried out in order to check for potential background reactions under the experimental conditions adopted for the catalytic tests (Table S3 in the Supporting Information). These include reactions without any catalyst (with and without base) as well as reactions using simple Ru, Cu, Ag, or Au salts as potential catalyst precursors. The conversions after 5 and 24 h were almost zero for all these control experiments, pointing out that the results summarized in Table 7 are not affected by any background reaction. Moreover, since under the experimental conditions employed herein, no conversion is observed using simple M(I) salts (M = Cu, Ag, and Au) as potential catalyst precursors, it is likely to be excluded that, in the case of heterometallic clusters, the catalytic activity is somehow related to the formation of M(I) compounds resulting from cluster breakdown. This further corroborates the opinion that catalysis should be obtained after cluster activation and not cluster decomposition.
In order to test the possibility of reusing the catalyst, it was at first attempted to again add some substrate at the end of the first catalytic run, but the conversion was rather low (see entries 1-2-R and 3-2-R in Table 8). This might be due to decomposition and/or a negative effect of the reaction product. Indeed, by performing the catalysis starting from a 1:1 mixture of the substrate (4-fluoroacetophenone) and product (4-F-α-methylbenzylalcohol), the conversion after 24 h was rather lowered as compared to the same experiment in the absence of 4-F-α-methylbenzylalcohol (compare entries 1-2 and 1-2-P in Tables 7 and 8). In order to remove the negative effect of the product, the reaction mixture was dried under reduced pressure at the end of the catalytic run, and the residue was washed with n-hexane. Then, the solvent and the substrate were added again, and a second catalytic run was carried out, resulting in 0% conversion (entry 1-2-R−H in Table 8), probably because of the decomposition of the catalyst during the work-up, even if the system had been kept under an inert atmosphere during all the manipulations.
The nature of the carbonyl species present in the reaction mixture at the end of the catalytic tests was investigated by combined IR, 1 H NMR, and ESI-MS analyses. In addition, clusters 2−4 were heated in i PrOH at a refluxing temperature, and the resulting products were spectroscopically analyzed. In all cases, complex mixtures of products were detected, including carbonyl hydride clusters. Among the different species, it was possible to identify [H 3 Ru 4 (CO) 12 ] − (11) 51,52 and [HRu 6 (CO) 18 ] − (12). 53 Other unidentified species were also present, probably also including heterometallic clusters. The fact that 2−4 were not present at the end of the catalytic tests indicated that they were transformed during catalysis. At the same time, it is possible to exclude that complete decomposition of the clusters to mononuclear species or metal nanoparticles occurs since cluster carbonyl species and hydride carbonyl clusters are still present, as indicated by IR, 1 H NMR, and ESI-MS (Figures S10−S19 in the Supporting Information). Thus, it seems that the catalyst precursors 2−4 are transformed into the active species during the induction period. The fact that mixtures of clusters are present at the end of the catalytic process (with no evidence of mononuclear species) suggests that cluster breakdown does not occur during the whole catalytic process. Therefore, this should involve only cluster transformations from precursors to active species and, eventually, to inactive species.
The formation of such complex mixtures of products and, in particular, the presence of hydrides are likely to be due to the use of a protic solvent such as i PrOH. Indeed, the thermal treatment under mild conditions (60−80°C) of 2−4 in aprotic solvents such as tetrahydrofuran (THF) or CH 3 CN resulted in the elimination of the coinage metal and formation of [Ru 6 (CO) 18 Figure  S7 in the Supporting Information). It is well known that 13 under protic conditions is transformed into 12. 53 Compound 13 resulted in 40% conversion after 24 h at 1% catalyst load, showing inferior performances to 1−3 under the same experimental conditions. This would suggest that the activation of heterometallic clusters 2 and 3 does not involve the formation of 13.   52 Analyses of C, H, and N were obtained with a ThermoQuest Flash EA 1112NC instrument. IR spectra were recorded on a PerkinElmer Spectrum One interferometer in CaF 2 cells. 1 H, 13 C{ 1 H}, 19 F{ 1 H}, and 31 P{ 1 H} NMR measurements were performed on a Varian Mercury Plus 400 MHz instrument. The proton and carbon chemical shifts were referenced to the nondeuterated aliquot of the solvent. The fluorine chemical shifts were referenced to external CCl 3 F. The phosphorus chemical shifts were referenced to external H 3 PO 4 (85% in D 2 O). Structure drawings have been performed with SCHAKAL99. 55

Synthesis of [NEt 4 ][AuRu 5 (CO) 19 ] ([NEt 4 ][4]). 4.4.1. From [NEt 4 ][HRu 3 (CO) 11 ] and Au(Et 2 S)Cl.
Au(Et 2 S)Cl (0.060 g, 0.185 mmol) was added slowly as a solid to a solution of [NEt 4 ][1] (0.300 g, 0.404 mmol) in CH 2 Cl 2 (15 mL). The resulting mixture was stirred at room temperature for 1 h. Then, the solvent was removed under reduced pressure, and the residue was washed with water (40 mL) and toluene (40 mL) and extracted with CH 2 Cl 2 (10 mL). The brown CH 2 Cl 2 solution was layered with n-pentane ( (15 mL). The resulting mixture was stirred at room temperature for 6 h. Then, the solvent was removed under reduced pressure, and the residue was washed with water (40 mL) and extracted with toluene (10 mL). The toluene solution was layered with n-hexane, affording crystals of 6·solv suitable for SC-XRD (yield 0.023 g, 4% based on Ru and 24% based on Cu). Then, the residue was extracted with CH 2 Cl 2 (10 mL), and the red solution was layered with n-pentane, affording crystals of [ (20 mL); the mixture was stirred at room temperature, and the reaction was monitored by IR spectroscopy. At the end of the reaction, the solution was evaporated to dryness, and the residue was washed with water (40 mL) and hot EtOH (40 mL) and then extracted with toluene (15 mL). The toluene solution was layered with n-hexane, affording crystals of HRu 3 (OH)(CO) 7 (PPh 3 ) 3 ·1.5toluene (9·1.5toluene) suitable for SC-XRD as the main product (yield 0.101 g, 16% based on Ru) together with a few crystals of Ru(CO) 3 Table S4 in the Supporting Information. The diffraction experiments were carried out on a Bruker Apex II diffractometer equipped with a PHOTON2 detector using Mo Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS). 56 Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F 2 . 57 Hydrogen atoms were fixed at calculated positions and refined with a riding model. All nonhydrogen atoms were refined with anisotropic displacement parameters unless otherwise stated.
4.10. Computational Details. Geometry optimizations were performed in the gas phase using the PBEh-3c method, which is a reparametrized version of PBE0 58 (with 42% HF exchange) that uses a split-valence double-zeta basis set (def2-mSVP) with relativistic ECPs for Ru, Ag, and Au 59 and adds three corrections that consider dispersion, basis set superposition, and other basis set incompleteness effects. 60 Single point calculations with the addition of the C-PCM solvation model were also carried out, considering dichloromethane as continuous medium. 61 The "restricted" approach was used in all the cases. Calculations were performed with ORCA 4.0.1.2. 62 The output, converted in a .molden format, was elaborated with the software Multiwfn, version 3.5. 63 The Cartesian coordinates of the DFToptimized structures are provided in a separate .xyz file.

Reactivity Experiment of [NEt 4 ][3] in i PrOH.
In a 10 mL two-neck round-bottom flask equipped with a condenser, [NEt 4 ] [3] (45 mg, 31 μmol) and i PrOH (7 mL) were stirred at reflux temperature under a nitrogen atmosphere, and then, 4-fluoroacetophenone (3.75 μL, 31 μmol) was added. The reaction mixture was stirred at reflux temperature under the nitrogen atmosphere. After 24 h, the solvent was removed under vacuum, and the crude of the reaction was analyzed by IR, 1 H NMR, and ESI-MS.

Reactivity Experiment of [NEt 4 ][1] in i PrOH.
In a 10 mL two-neck round-bottom flask equipped with a condenser, [NEt 4 ][1] (74.5 mg, 100 μmol) and i PrOH (7 mL) were stirred at reflux temperature under a nitrogen atmosphere, and then, 4-fluoroacetophenone (12.10 μL, 100 μmol) was added. The reaction mixture was stirred at reflux temperature under the nitrogen atmosphere. After 24 Inorganic Chemistry pubs.acs.org/IC Article h, the solvent was removed under vacuum, and the crude of the reaction was analyzed by IR, 1 H NMR, and ESI-MS.

Transfer Hydrogenation of 4-Fluoroacetophenone with [NEt 4 ][2]
with Recycling of the Catalytic Species. In a 10 mL two-neck round-bottom flask equipped with a condenser, [NEt 4 ][2] (3 μmol, 1% mol/mol) and i PrOH (5 mL) were stirred at reflux temperature under a nitrogen atmosphere for 5 min. Then, the substrate 4-fluoroacetophenone (36.5 μL, 300 μmol) was added. After 24 h of reaction, a sample was taken; the solvent was removed under vacuum, and the solid was washed with three 5 mL aliquots of hexane. Right after the removal of the solvent under vacuum, i PrOH (5 mL) and 4-fluoroacetophenone (36.5 μL, 300 μmol) were added again, and the flask was put at reflux temperature under the nitrogen atmosphere. The last sample was taken at 48 h of reaction. Aliquots (100 μL) were diluted with CDCl 3 (0.5 mL) and conversions were determined by 19F NMR spectroscopy.

Reactivity Experiment of [NEt 4 ][2] in CH 3 CN. A solution of [NEt 4 ][2]
(0.171 g, 0.121 mmol) in CH 3 CN (15 mL) under a nitrogen atmosphere was heated at 80°C for 6 h. Then, the solvent was removed under reduced pressure, and the residue was washed with water (40 mL) and toluene (20 mL), and the product was extracted with acetone (10 mL). Crystals of [NEt 4 ] 2 [12]· CH 3 COCH 3 were obtained by slow diffusion of n-hexane on the acetone solution.

Reactivity Experiment of [NEt 4 ][4] in THF.
A solution of [NEt 4 ][4] (0.171 g, 0.125 mmol) in THF (15 mL) under a nitrogen atmosphere was heated at 66°C for 12 h. Then, the solvent was removed under reduced pressure, the residue was washed with water (40 mL) and toluene (20 mL), and the product was extracted with CH 2 Cl 2 (10 mL